This invention relates to a device for supplying electrical energy to a load, comprising:
a) first and second input terminals adapted to be connected to first and second poles of a first DC voltage source;
b) third and fourth input terminals adapted to be connected to first and second poles of a second DC voltage source;
c) first and second output terminals adapted to be connected to the load;
d) first and second electrical connections connecting the first and second input terminals to the first and second output terminals, respectively;
e) third and fourth electrical connections connecting the third and fourth input terminals to the first and second output terminals, respectively;
f) a switch means included in at least one of the first and second electrical connections, said switch means comprising a first electrode connected to one of the first and second input terminals and a second electrode connected to the first output terminal, said switch means being connected to established an electrically conducting path between its first and second electrodes if a first DC voltage source is connected to the first and second input terminals and unless a second DC voltage source producing a voltage exceeding the voltage produced by the said first DC voltage source is connected to the third and fourth input terminals;
g) a selectively conductive element included in at least one of said third and said fourth electrical connections, said selectively conducting element being in an electrically conducting state when the second DC voltage source is connected to the third and fourth input terminals with a predetermined polarity and in a non-conducting state when the second DC voltage source is disconnected and the first DC voltage source is connected to the first and second input terminals.
A device of the kind set forth is disclosed in a publication from Phillips Components with the title "OM1016 I.sup.2 C Evaluation Board", see page 12 the section "Clock/Calendar". In the known device the switch means is formed by a diode. A disadvantage of this arrangement is that a relatively high voltage drop always occurs over a diode, which causes the useful voltage available for the load to be always below the nominal voltage produced by the first DC voltage source. This is particularly inconvenient when the first DC voltage source produces only a small voltage, e.g. when it is a small battery. In such a case the voltage drop across the diode constitutes a considerable portion of the voltage produced by the voltage source. Moreover, a relatively lot of power is dissipated in such a diode, causing a decrease in the useable capacity of the battery, which is also particularly disadvantageous when the voltage produced by that battery is relatively small.